Driving assistance systems and active safety systems that are being designed for the future must be capable of influencing the steering system by means of an electronic controller. Degrees of freedom in the steering system generally comprise the steering position (angular position of the wheels) and the steering sensation (manual/steering wheel torque). The two degrees of freedom may each be controlled by suitable actuators, generally referred to as electronically controllable control system (ECS). Examples of ECS systems in which the steering position can be controlled independently of driver inputs are Active Front Steering (AFS) systems, rear axle steering (RAS) systems, and steer-by-wire systems in which steering commands are passed on to an actuator exclusively by electronic means.
If a driver does not exert any influence on the lateral behavior of the vehicle, that is to say he removes his hands from the steering wheel, steering position control may be carried out by means of additional steering systems in which the steering torque is controlled, such as for example in an electric power assisted steering (EPAS) system. EPAS systems or combinations, such as the combination of an AFS/steer-by-wire system with an EPAS system, are capable of carrying out a steering sensation/torque control process. In this case, the driver also applies a certain steering torque, which has to be taken into account in the control architecture. Furthermore, an electro-hydraulically assisted steering (EHPAS) system can assist the steering torque, but this usually takes place in a control range which is restricted compared to the EPAS system.
In the future development of vehicles, it will be necessary to cover different steering functionalities simultaneously, even under circumstances in which the control loops for the steering position and the steering torque issue commands simultaneously.
Each individual system checks its activation with respect to specific driving states and/or driver inputs. However, a particular problem arises when the systems request activation simultaneously. Furthermore, although each individual system monitors the angle request or torque request which is generated by the respective system, the resulting overall value for the angle or torque may exceed a specific limiting value which is predefined by the controllability on the part of the driver. Incorrect arbitration and limitation may give rise to uncomfortable steering behavior, and in the worst case, may even bring about a situation in which an average driver may lose control over the steering system.
Known approaches to a solution include a Multi-Input Multi-Output (MIMO) concept in which a plurality of control loops are coupled to one another and are controlled by a central controller. Therefore, different functionalities may be controlled by such a concept given the maximum functionality of the individual systems. In the case of simultaneous activation of the systems, it may be possible for disadvantages to occur, such as during the management of simultaneous requests to a steering system. It may be that non-linearities are not all covered by the MIMO concept. Additionally, the establishment of a central MIMO controller increases the adjustment complexity considerably. If a system is added or removed, the entire MIMO controller must be readjusted. Furthermore, the robustness of a central MIMO controller, with respect to variation of the parameters, for example due to the aging process, is less than it would be for a case in which there are several control loops with a single output.
There is a need for a device and method for controlling a steering system in a vehicle for which different requests to the steering system may be reconciled with one another as a function of a state of the vehicle and a current driving style.